Polarizing Microscope in the Examination of Organic Bodies. 449 



very strongly upon the light coming from the polarizer, rendering 

 the held too luminous, and imparting to it too intense a colour. 

 In these cases, the difference between the intense colour of the 

 field and the but slightly differing colour of a very delicate and 

 feebly polarizing object is so difficult to detect, in many instances, 

 that the use of the entire process is very doubtful. On the other 

 hand I cannot too strongly recommend for this purpose the use 

 of very thin plates of mica instead of the selenite. Mica being 

 so tough and readily split, renders it an easy matter to separate 

 such thin lamina? that, when inserted between the polarizer and 

 the object, the field is not coloured, but merely appears more 

 luminous ; so that when a very thin plate is used, it becomes 

 dark-grey, and the thicker the plates, the whiter the colour. If 

 a doubly refracting object is then placed under the microscope ; 

 according to its position as regards the plate of mica, it becomes 

 more or less brilliantly white, or more or less dark-black. These 

 so strongly contrasted degrees of brilliancy of the object, and 

 their difference from the uniform grey colour of the field, are 

 more easily and distinctly perceptible to the eye than differences 

 of colour ; so that in many cases in which, when the selenite 

 plate is used, a doubtful result is obtained, the use of the plate 

 of mica is successful. A satisfactory result, however, is only ob- 

 tained when the thickness of the plate of mica and its corre- 

 sponding action upon the polarized light coincide with the 

 intensity of the doubly refracting power of the object. The 

 more feeble the latter, the thinner must be the plate of mica, and 

 the less luminous it must render the field. A series of four or five 

 plates, the thinnest of which brightens the field but little, whilst 

 the thickest produces considerable brightness but no colour, is 

 sufficient for all cases. 



In a paper upon the examination of vegetable tissues by the aid 

 of polarized light (' Ann. Nat. Hist/ 1858, vol. i.), I have shown 

 that different vegetable elementary organs, having the same struc- 

 ture but of different chemical composition, are opposed in their 

 action upon polarized light, one exhibiting positive, the other 

 negative colours. To recognize this relation, the diminution of 

 the grey colour of the object produced by the plate of mica, to 

 a more or less deep black, or its augmentation to white, may be 

 used ; and the application of the mica plates deserves the pre- 

 ference in all those cases in which the object acts but feebly 

 upon polarized light. In the case of objects, however, which 

 possess this property to a considerable degree, the selenite plates 

 are preferable, because the contrast of the brilliant comple- 

 mentary colours, apparent under these circumstances, is very 

 striking. To produce these colours clearly, a series of selenite 

 plates of increasing thicknesses should also be kept, because, 



Ann. §■ Mag. N. Hist. Ser. 3. Vol. iv. 29 



